1. Field of the Invention
The present invention relates to a thermal transfer image receiving sheet, more specifically to a thermal transfer image receiving sheet, for a thermal transfer printing, which is capable of forming a recording image excellent in color density, sharpness and various toughness, especially in durabilities such as light resistance, fingerprint resistance and plasticizer resistance or the like.
2. Description of the Related Art
Heretofore, various thermal transfer methods have been known. Among these methods, there has been proposed a method wherein a sublimable dye is used as a recording agent and is carried on a substrate sheet such as paper and plastic film to prepare a thermal transfer sheet, and various full color images are formed, by using the thermal transfer sheet, on a thermal transfer image receiving sheet which is capable of being deposited with a sublimable dye, for example, a thermal image receiving sheet having a dye receptor layer on a paper or a plastic film.
In such a case, a thermal head of a printer is used as heating means so that a large number of color dots of three or four colors are transferred onto the thermal transfer image receiving sheet under heating in a very short period of time. As a result, a full color image of an original is reproduced by using the multi-color dots. The thus formed images are very clear or sharp and are excellent in transparency, since the dyes are used therein as a colorant. Accordingly, these images are excellent in half tone reproducibility and gradation characteristic, and are substantially the same as the images formed by the conventional offset printing and gravure printing. Further, when the image forming method mentioned above is carried out, there can be formed images of high quality which are comparable to full color photographic images.
In order to effectively perform the thermal transfer method described above, of course, the structure of the thermal transfer sheet is important. In addition, the structure of the image receiving sheet for forming the image is also important. As examples of the conventional art regarding to the image receiving sheet described above, for example, Japanese Patent Laid-Open Publication Nos. SHO 57-1639370 and SHO 60-25793 disclose a formation of the dye receptor layer onto the substrate sheet by using polyester type resins, vinyl type resins such as polyvinyl chloride resin, polycarbonate resins, polyvinyl butyral type resins, acrylic resins, cellulose type resins, olefin type resins, polystyrene type resin or the like.
In the thermal transfer image receiving sheet described above, dye transferring sensitivity of the dye receptor layer and various durabilities or preserving stability of the thus formed image greatly depend on the resin constituting the dye receptor layer. In particular, light resistance of the formed image greatly depends on an amount ratio and chemical structures of respective resins in a resin composition constituting the receptor layer, so that it is required to select an optimum resin composition. As an example of the resin or resin composition excellent in light resistance, aromatic polycarbonate resin is available. For example, Japanese Patent Laid-Open Publication Nos. SHO 62-169694 and HEI 5-131758 disclose such various aromatic polycarbonate resins.
Further, in order to improve the transferring sensibility of the dye, it is sufficient to improve diffusing property of the dye. As a countermeasure to improve the diffusing property of the dye, there has been proposed various methods, for example, a method in which a resin having a low glass-transition temperature (Tg) is used as the resin constituting the receptor layer, or a method in which a plasticizer is added into the receptor layer. Japanese Patent Laid-Open Publication No. HEI 2-301487 disclose a method of lowering the Tg by copolymerization. Japanese Patent Laid-Open Publication Nos. SHO 60-19138, HEI 2-80291 and SHO 62-202791 disclose a method of improving the dye transferring sensitivity of the aromatic polycarbonate resin by adding the plasticizer or a resin having a low Tg.
However, the prior art techniques described above arise the following problems.
Namely, as to polycarbonate resin derived from 2,2-bis(4-hydroxyphenyl)propane (i.e., bisphenol A) which is deemed to be the most popular and suitable in most of the Official Gazettes of the prior art techniques described above, this resin being composed of only an unit 1 represented by the following formula 1: 
and, as to a copolymer of polycarbonate resin disclosed in Japanese Patent Laid-Open Publication No. HEI 2-301487, these resins have insufficient solubility with respect to solvent. Therefore, when these resins are used for manufacturing the thermal transfer image receiving sheet, it is required to dissolve these resins with the by use of chlorinated solvents such as methylene chloride, trichloromethane or the like, thus resulting in harmful deterioration of working environments for workers.
In contrast, as an example of an aromatic polycarbonate resin having a sufficient solubility and enabling a coating work with the use of non-halogenated type hydrocarbon solvents such as ketone type, toluene type, or mixture-solvent thereof, there has been proposed a polycarbonate resin which is derived from 1,1-bis(4-hydroxyphenyl)cyclohexane (i.e., bisphenol Z), this polycarbonate resin being represented by the following formula 5: 
[in formula 5, xe2x80x9cpxe2x80x9d denotes integer]
and also proposed a polycarbonate resin derived from 2,2-bis(4-hydroxy-3-methylphenyl)propane (i.e., bisphenol C), this resin being composed of only an unit 2 represented by the following formula 2: 
Further, Japanese Patent Laid-Open Publication No. HEI 5-131758 discloses various copolymers of polycarbonate resins each having sufficient solubility.
However, there were caused problems that these easily dissolvable polycarbonate resins are inferior to those derived from bisphenol A in light resistance, or that aromatic dihydroxy compounds to be used as the raw material of the easily dissolvable polycarbonate resins has an industrial disadvantage because they are expensive in comparison with bisphenol A.
In addition, these aromatic polycarbonate resins generally have high glass-transition temperatures (Tg). Therefore, in order to obtain sufficient transferring sensitivity of the dye, there has been considered a method in which aforementioned dihydroxy compound to be used as the raw material is selected and copolymerized with bisphenol A thereby to lower the Tg of the polycarbonate per se, or a method in which the receptor layer is plasticized by further adding the plasticizer or the resin having a low Tg thereby to improve the transferring property and the diffusing property of the dye.
However, when the Tg is lowered by the copolymerization, molecular structure of the polymer is changed, so that an excellent light resistance that are inherent to bisphenol A type polycarbonate resin are liable to be deteriorated. In addition, in order to sufficiently improve the transferring property and diffusivity of the dye, it is required to set the Tg to an extremely low level, so that there may be caused a problem that a fixing property of the dye is deteriorated and blur of image are liable to occur after the printing operation, or a problem of a difficulty in selecting suitable dihydroxy compound capable of being copolymerized with bisphenol A as well as to reduce the material cost.
As another method of improving the transferring property and the diffusing property of the dye, as disclosed in the aforementioned Official Gazette, there has been proposed a method in which the plasticizer or the resin having a low Tg is added into the resin for constituting the receptor layer.
In this method, all of the transferring property, diffusing property and fixing property of the dye can be easily controlled so as to meet with the required levels by adjusting the addition amount of the plasticizer or resin having a low Tg. In particular, in a case of polycarbonate resin having a high Tg (e.x., bisphenol A type has a Tg of about 150xc2x0 C.), this resin is preferable because the properties of the receptor layer can be controlled in a broader range by adjusting an addition amount of the resin having a low Tg.
However, when physical properties or chemical structures of the plasticizer and the low-Tg resin to be added are not optimum, the compatibility of these compounds with respect to aromatic polycarbonate resin is damaged thereby to arise the following problems.
(1) Namely, after the formation of the dye receptor layer, the plasticizer and the low-Tg resin are bled out with time to change the transferring property and diffusing property of the dye, so that the recording sensitivity will be also changed with time.
In a case where the compatibility is worse, the fixing property of the dye will be insufficient, so that there may be caused a problem that the blur of image will occur during the recording procedure, or there may be a case where the printing operation per se will become impossible due to occurrence of tacks in the dye receptor layer.
(2) Even if there is no abnormal defect at the time of the recording procedure, when the recorded image is preserved, in particular, in a high temperature condition, the recorded image will be blurred due to the bleeding-out of the dye after the recording operation.
These problems are particularly liable to arise when a comparatively large amount of the plasticizer or the low-Tg resin is added.
An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art described above and to provide a thermal transfer image receiving sheet to be used in a thermal transfer printing method using a sublimable dye, the sheet being capable of forming a recording image excellent in color density, sharpness and various toughness, especially in light resistance and also capable of being easily manufactured by utilizing an ordinary coating device and by using a non-halogenated type organic solvents such as ketone type, toluene type, or a mixed solvent thereof.
This and other objects of the present invention can be achieved by providing, in one aspect, a thermal transfer image receiving sheet comprising a substrate sheet and a dye receptor layer disposed on at least one surface of the substrate sheet, wherein the dye receptor layer comprises polycarbonate resin of a random copolymer having a main chain which comprises, as essential units, an unit 1 represented by the following formula 1 and an unit 2 represented by the following formula 2, an amount ratio of the unit 1 being not more than 70 mol % based on a total amount of the unit 1 and the unit 2, the polycarbonate resin having a glass transition temperature of not less than 125xc2x0 C. and being dissolvable in a general solvent: 
In another aspect of the present invention, there is provided a thermal transfer image receiving sheet comprising a substrate sheet and a dye receptor layer disposed on at least one surface of the substrate sheet, wherein the dye receptor layer comprises (1) polycarbonate resin of a homopolymer having a main chain which comprises an unit 2 represented by the following formula 2 and (2) at least one additive selected from the group consisting of phthalic acid type plasticizers, phosphoric ester type plasticizers, polycaprolactones and polyester plasticizers: 
In the respective thermal transfer image receiving sheet as described above, it is preferable to use polycarbonate resin which is dissolvable in a general solvent. It is preferable that the general solvent is a non-halogenated type organic solvent.
Furthermore, it is preferable that the dye receptor layer comprises at least one additive selected from the group consisting of phthalic acid type plasticizers, phosphoric ester type plasticizers, polycaprolactones and polyester plasticizers. Each of these additives preferably has a melting or freezing point of not less than 60xc2x0 C.
In addition, as the phthalic acid type plasticizer, dicyclohexyl phthalate is preferably used.
Furthermore, it is preferable that the phosphoric ester type plasticizer is at least one compound selected from the group consisting of non-halogenated phosphoric esters and non-halogenated condensed phosphoric esters.
In addition, as the non-halogenated phosphoric ester, it is preferable to use a compound represented by the following formula 3: 
[where each of R1 and R2 denotes hydrogen atom, alkyl group or substituted alkyl group].
In addition, as the non-halogenated condensed phosphoric ester, it is preferable to use a compound represented by the following formula 4: 
[wherein each of R3 and R4 denotes hydrogen atom, alkyl group or substituted alkyl group].
Still further, it is preferable that the dye receptor layer further comprises aromatic saturated polyester resin.
Furthermore, it is preferable that the dye receptor layer further comprises at least one release agent selected from the group consisting of silicone oils and hardened products of the silicone oils.
In addition, as the hardened product of the silicone oil, it is preferable to use at least one compound selected from the group consisting of hardened products of addition polymerization silicones and hardened products of carbinol-modified silicones hardened with isocyanate compounds.
According to the thermal transfer image receiving sheet having aforementioned structure, the dye receptor layer is formed from the polycarbonate resin having a specified chemical structure as described in the present invention. Therefore, there can be printed images excellent in color density, sharpness or clarity and toughness, particularly in light resistance. In addition, there can be provided a thermal transfer image receiving sheet which can be easily manufactured by using an ordinary coating apparatus in which non-halogenated type organic solvents such as ketone type solvent, toluene type solvent or blended solvent thereof are used.